1. Field of the Invention
The present invention relates to a display device and manufacturing methods therefor, and more particularly, relates to an electrophoretic display device wherein a display is created by causing the migration of charged color particles in a liquid by a voltage applied to electrodes, and a manufacturing method therefor.
2. Description of the Related Art
Recently, concomitant with the progress of information devices, data volume of various information has been increasing rapidly, and in addition, output of information has been performed in various forms. In general, the output of information can be roughly classified into a screen display using a display device, such as a cathode ray tube or a liquid crystal panel, and a hard-copy display printed on paper by a printer or the like. In the screen display, the need for a thinner display device having a low power consumption has been growing, and above all, a liquid crystal display device has been actively developed and commercialized as a display device which can fulfill the need describe above. However, for the current liquid crystal display devices, problems have been encountered in that letter images displayed on the screen are difficult to view depending on a viewing angle or reflection and, in addition, eyestrain caused by a flickering light source, low luminance, and the like has not been satisfactorily solved. In addition, a screen display using a cathode ray tube has a similar or superior contrast and luminescence compared to a liquid crystal display; however, since a flicker may be generated in a screen display device, the display quality cannot be similarly compared to the hard-copy display described later. In addition, the device using a cathode ray tube is large and heavy, and hence, the portability thereof is significantly inferior.
It has been believed that the hard-copy displays would have disappeared due to computerized information processing; however, a significant amount of hard-copy output has been used in practice. The reason for this is that when information is displayed on a screen, in addition to the problems relating to the display quality described above, the resolution is generally up to 120 dpi (dots per inch) and is considerably inferior to a printout on paper (generally, 300 dpi or more). Accordingly, eyestrain is larger for the screen display compared to the hard-copy display. As a result, even when information can be viewed by the screen display, a hard-copy output is frequently performed. In addition, another important reason the hard-copy display is used even though the screen display can be used is that once hard-copy information is output, a great deal of information can be laid out without being restricted by the size of a screen as in the case of the screen display, and the information thus laid out can be rearranged and can be read one by one without performing a complicated device operation. Furthermore, the hard-copy display has a superior portability since no energy is required for retaining the display, and as long as the volume of information is not significantly large, the information can be read at any time and at any place.
As described above, as long as a movie display or a frequent rewrite is not required, the hard-copy display has various features different from the screen display; however, there is a shortcoming in that a large amount of paper is consumed for a hard copy display. Accordingly, in recent years, development of rewritable recording media (recording media in which a number of cycles of recording and erasing a high visibility image can be performed, and no energy is required for retaining the image thereon) has been actively advancing. A rewritable third display method which successively possesses the features of the hard-copy display described above is referred to as a paper-like display.
Requirements for the paper-like display are rewritable, no energy consumption for retaining an image or a significantly small energy consumption therefor (memory retaining characteristic), superior portability, superior display quality, and the like. As a display method which is currently regarded as the paper-like display, for example, there may be mentioned reversible display media using a matrix system formed of a low molecular resin or a high molecular resin (disclosed in, for example, Japanese Unexamined Patent Application Publication Nos. 55-154198 and 57-82086), which performs recording/erasing by using a thermal printer head. The system described above is used for display portions of some prepaid cards; however, there are problems in that the contrast is not so high, the number of repeatable cycles of recording/erasing is relatively small, such as 150 to 500 times, and the like.
As another display method which can be used as the paper-like display, an electrophoretic display device (U.S. Pat. No. 3,612,758) invented by Harold D. Lees, et al. has been known. In addition to the patent mentioned above, Japanese Unexamined Patent Application Publication No. 9-185087 also discloses an electrophoretic display device. FIGS. 7A and 7B are views showing the structure and the operation principle of these electrophoretic display devices mentioned above. This display device includes a pair of substrates 1 and 2 disposed with a predetermined spacing therebetween, and electrodes 3 and 4 formed on the substrates 1 and 2, respectively. The display surface is a side indicated by the arrow B, and the electrode 4 provided at the display surface side is transparent. In addition, between the two substrates 1 and 2, there are provided a number of charged color particles 5 which are positively charged and are also colored in this case; an insulating liquid 6 which is colored by a dye dissolved therein so as to have a different color from that of the charged color particles; and partitions 13 which divide the spacing described above into a number of sections in the direction along the surface of the substrate so as to prevent the charged color particles from being localized and to define the spacing between the substrates.
In the display device described above, as shown in FIG. 7A, when a negative voltage is applied to the electrode 3 shown at the lower side in the figure, and in addition, a positive voltage is applied to the electrode 4 at the upper side in the figure, the charged color particles 5 which are positively charged are gathered so as to cover the electrode 3 at the lower side, and when this display device is viewed along the B direction in the figure, a display having the same color as that of the insulating liquid 6 is created.
In contrast, as shown in FIG. 7B, when a positive voltage is applied to the electrode 3 shown at the lower side in the figure, and in addition, a negative voltage is applied to the electrode 4 at the upper side in the figure, the charged color particles 5 which are positively charged are gathered so as to cover the electrode 4 at the upper side, and when this display device is viewed along the B direction in the figure, a display having the same color as that of the charged color particles 5 is created. When this operation described above is performed in each pixel unit, an optional image can be displayed by a number of pixels.
The display device shown in FIGS. 7A and 7B creates a display by causing the migration of the charged color particles in the direction perpendicular to the surface of the substrate in accordance with a voltage applied between the electrodes provided on the pair of substrates opposing each other. In addition to this display device, a display device disclosed in Japanese Unexamined Patent Application Publication Nos. 49-5598 and 11-202804 has been proposed in which a pair of electrodes, i.e., a first display electrode and a second display electrode, is disposed on the same substrate, and charged color particles are moved in the direction parallel to the substrate when viewed from an observer side. This display device described above creates a display by causing the migration of the charged color particles in a transparent insulating liquid in parallel to the surface of the substrate and between the first display electrode and the second display electrode in accordance with a voltage applied therebetween.
In this horizontal migration type electrophoretic display device, a transparent insulating liquid is used in many cases, and when viewed from the observer side, the first display electrode and the second display electrode show different colors from each other, and one of the color of the display electrodes is the same as that of the migrating particles. For example, in the case in which the first display electrode is black, the second display electrode is white, and the migrating particle is black, when the migrating particles are gathered at the first display electrode side, the second display electrode is exposed so that a white color is viewed, and when the migrating particles are gathered at the second display electrode side, a black color which is the color of the migrating particle is viewed.
In addition, recently, an electrophoretic display device using microcapsules has been proposed (U.S. Pat. No. 2,551,783). In FIG. 6, a cross-sectional view of the electrophoretic display device using the microcapsules is shown. Charged color particles 5 which are colored and an insulating liquid 6 which is colored so as to have a different color from that of the charged color particle are enclosed in transparent containers 8 having a diameter of approximately 50 xcexcm, and these containers 8 are applied to a substrate 1, thereby forming a display device. In order to create an image display, electrodes 3 and 4 are provided at the top and the bottom of this microcapsules 8, and as in the case of the conventional electrophoretic display method, a voltage is applied between the electrodes 3 and 4, whereby the color of the charged color particles 5 or the color of the insulating liquid 6 can be displayed.
As for the features of the microcapsule type electrophoretic display device, since the charged color particles and the insulating liquid (that is, a dispersing liquid for migration) can be disposed by merely applying the microcapsules to the substrate, compared to the conventional electrophoretic display device, injection steps for the charged color particles and the insulating liquid are not required, and hence, formation of the display device can be easily performed. In addition, since partitions are not particularly required, for example, displacement or damage of partitions caused by warping of the substrate does not occur, and the flexibility of the display device can be increased.
However, in the conventional microcapsule type electrophoretic display device, since the microcapsules are mixed with a binder and are then applied to the substrate together with the binder, some microcapsules overlap each other or are disposed at a place at which the electrodes are not provided, whereby there are some microcapsules which do not serve to create a display.
In addition, in order to create a color display, microcapsules having different colors must be regularly disposed, and in the case described above, a printing plate for printing a predetermined shape with the microcapsules mixed with a binder is required, whereby there is a problem in that the manufacturing cost is increased.
As a method of disposing microcapsules in an area at which electrodes are provided, in addition to the printing method described above, as disclosed in Japanese Unexamined Patent Application Publication No. 2000-35769, a method has been proposed in which by using an ink-jet type ejection head provided with a nozzle having a diameter through which one microcapsule is allowed to pass, the microcapsules are sequentially ejected on desired positions on a substrate. However, in the method described above, in addition to the difficulty of ejecting the microcapsules one by one, there is another problem in that it is difficult to accurately fix the microcapsules at the predetermined positions so as to be adjacent to each other. The reason for this is that when the microcapsule thus ejected reaches the substrate, the microcapsule is brought into contact with a microcapsule adjacent thereto which is previously disposed, so that the predetermined position for each microcapsule was displaced.
As described above, since it has been difficult to regularly and accurately dispose the microcapsules so as to form a monolayer, the microcapsules which are prepared cannot be efficiently used, and in addition, the difficulty described above has been one of major obstacles to the formation of a color display device.
In addition, since the microcapsules has a spherical shape, when a monolayer thereof is formed, gaps are formed between the microcapsules adjacent to each other, and as a result, an effective display area is decreased corresponding to the gaps thus formed, whereby the contrast of the display is decreased. As a method for avoiding the problem described above, it may be considered that microcapsules in a flat shape are formed by compression so as not to form the gaps between the capsules. When the microcapsules can be formed in a flat shape, the driving voltage can also be decreased.
However, since a microcapsule located at the edge portion of the electrode has no microcapsule adjacent thereto, when the microcapsules are compressed, a part of the microcapsule located at the edge portion of the electrode is extended outside from the electrode and is adversely influenced by an adjacent electrode, and as a result, the contrast may also be decreased in some cases. In order to form flat microcapsules without extending outside from the electrode, the microcapsules must be accurately disposed at a central portion of the electrode beforehand; however, in the case described above, a highly accurate alignment technique is required. In particular, as is the case in which a horizontal migration type display is performed using the microcapsules, the phenomenon described above may become a major problem when microcapsules having a size approximately equivalent to that of a pixel are used.
The present invention was made to solve the problems described above, and an object of the present invention is to provide a display device for use in a microcapsule type electrophoretic display apparatus and a manufacturing method therefor, in which microcapsules can be aligned to form a monolayer structure, and accordingly, the microcapsules can be efficiently used. In addition, another object of the present invention is to provide a display device and a manufacturing method therefor, in which a color display can be created, positioning of the microcapsules can be easily performed, and as a result, the contrast is improved.
That is, a display device in accordance with one aspect of the present invention comprises a substrate, an insulating liquid, charged color particles dispersed in the insulating liquid, a first electrode formed on the substrate, and a second substrate, in which the charged color particles migrate toward the first electrode or the second electrode by applying a voltage therebetween so that a display is created. In the display device of the present invention described above, the insulating liquid and the charged color particles are enclosed in light transmissive walls so as to form microcapsules, and the microcapsules are aligned and are enclosed in fibers composed of the light transmissive resin.
One line of aligned microcapsules is preferably enclosed in the light transmissive resin fiber.
In addition, the length of the microcapsule in the direction parallel to the substrate is preferably larger than the length of the microcapsule in the direction perpendicular to the substrate.
The fibers enclosing the microcapsules are preferably aligned on the substrate.
In accordance with another aspect of the present invention, a method for manufacturing an electrophoretic display device including a substrate; an insulating liquid; charged color particles dispersed in the insulating liquid; and a first electrode and a second electrode for applying a voltage on the charged color particles provided on the substrate; in which the charged color particles migrate toward the first electrode or the second electrode by a voltage applied therebetween so that a display is created. The method comprises a preparing step of preparing microcapsules containing the insulating liquid and the charged color particles therein, a forming step of forming fibers each comprising a light transmissive resin and the microcapsules which are aligned and enclosed in the light transmissive resin; and a disposing step of disposing the fibers on the substrate provided with at least one of the first electrode and the second electrode.
The forming step described above is preferably performed by extruding the microcapsules with the light transmissive resin from a nozzle so that the microcapsules are aligned and enclosed in the light transmissive resin.
The manufacturing method described above preferably further comprises a step of compressing the microcapsules into a flat shape, and a step of curing the light transmissive resin for forming the fibers which enclose the microcapsules in the flat shape.
In accordance with another aspect of the present invention, microcapsules for use in electrophoretic display comprise charged color particles, an insulating liquid in which the charged color particles are dispersed, and light transmissive walls each enclosing the charged color particles and the insulating liquid, wherein microcapsules in an aligned state are enclosed in a light transmissive resin, and the light transmissive resin is in the form of a fiber.